DREYFUS INSTITUTIONAL MONEY MARKET FUND INC
N-30D, 1999-08-26
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Dreyfus

Institutional

Money Market Fund

SEMIANNUAL REPORT June 30, 1999

(reg.tm)






<PAGE>

The  views  expressed herein are current to the date of this report. These views
and  the  composition  of the fund's portfolio are subject to change at any time
based on market and other conditions.

   * Not FDIC-Insured  * Not Bank-Guaranteed  * May Lose Value

Year 2000 Issues (Unaudited)

The fund could be adversely affected if the computer systems used by The Dreyfus
Corporation and the fund's other service providers do not properly process and
calculate date-related information from and after January 1, 2000. The Dreyfus
Corporation is working to avoid Year 2000-related problems in its systems and to
obtain assurances from other service providers that they are taking similar
steps. In addition, issuers of securities in which the fund invests may be
adversely affected by Year 2000-related problems. This could have an impact on
the value of the fund's investments and its share price.



<PAGE>

                                 Contents

                                 THE FUND
- --------------------------------------------------

                             2   Letter from the President

                             3   Discussion of Fund Performance

                             6   Statements of Investments

                            11   Statement of Assets and Liabilities

                            12   Statement of Operations

                            13   Statement of Changes in Net Assets

                            14   Financial Highlights

                            16   Notes to Financial Statements

                                 FOR MORE INFORMATION
- ---------------------------------------------------------------------------

                                 Back Cover

<PAGE>


                                                                       The Fund

                                                          Dreyfus Institutional

                                                              Money Market Fund


LETTER FROM THE PRESIDENT

Dear Shareholder:

We are pleased to present this semiannual report for Dreyfus Institutional Money
Market Fund, covering the six-month period from January 1, 1999 through June 30,
1999.  Inside,  you' ll find valuable information about how the fund was managed
during  the  period,  including  a  discussion  with  senior  portfolio manager,
Patricia A. Larkin.

After  remaining  relatively  steady during the first quarter of 1999, yields on
money  market  securities  generally  rose  in the second quarter in response to
expectations  that  the  Federal  Reserve  Board would raise short-term interest
rates  at  their June meeting. On June 30, the Federal Reserve raised rates amid
stronger-than-expected  global and domestic economic growth. Their objective was
to forestall a potential resurgence of inflationary pressures.

We  appreciate  your confidence over the past six months, and we look forward to
your  continued  participation  in  Dreyfus  Institutional  Money  Market  Fund

Sincerely,


Stephen E. Canter

President and Chief Investment Officer

The Dreyfus Corporation

July 15, 1999





<PAGE>

DISCUSSION OF FUND PERFORMANCE

Patricia A. Larkin, Senior Portfolio Manager

How did Dreyfus Institutional Money Market Fund  perform during the period?

For the six-month period ended June 30, 1999, Dreyfus Institutional Money Market
Fund produced an annualized yield of 4.49% for its Money Market Series and 4.11%
for  its  Government  Securities  Series.  Taking  into  account  the  effect of
compounding,  the  annualized effective yield was 4.59% for its Money Market and
4.19%  for  its  Government Securities Series.(1) The fund's Money Market Series
provided  a total return of 2.25%,(2) compared to the Lipper Institutional Money
Market Funds category average total return of 2.31%, while the fund's Government
Series provided a total return of 2.06%,(2) compared to the Lipper Institutional
U.S.  Treasury Money Market Funds category average total return of 2.14% for the
same period.(3)

What is the fund's investment approach?

There  are many factors we consider in managing the fund. We closely monitor the
outlook  for  growth  and  inflation.  We  follow  overseas developments for any
influence  they  may  have  on  the domestic economy. The posture of the Federal
Reserve  Board  (the  "Fed") is a key determinant in our decision on how best to
structure the fund.

In  addition,  we actively manage the average maturity of the fund in an attempt
to  take  advantage  of  expected  interest rate changes based upon our economic
outlook.  If  we  believe  that  interest rates will fall, we typically lengthen
average maturity to lock in the then-current rates. Conversely, in a rising rate
environment,  we  typically  shorten  maturities  to  be  able  to  reinvest  at
anticipated higher rates in the future.

                                                             The Fund



<PAGE>

DISCUSSION OF FUND PERFORMANCE (CONTINUED)

As  a  money  market fund, the fund only buys securities rated in one of the two
highest rating categories for debt obligations, or of comparable credit quality.
The  fund must also maintain an average dollar-weighted fund maturity of 90 days
or  less  and  may buy only securities with remaining maturities of 13 months or
less.

What other factors influenced the fund's performance?

Last fall, in the wake of Asian market turmoil, the Open Market Committee of the
Fed cut short-term interest rates three times in an attempt to provide liquidity
and  improve  investor  confidence.  Since  then,  there have been concerns that
global  and  domestic  factors  might  push  the  United  States economy towards
unsustainable growth.

As of June 30, 1999, Asian economies appear to have stabilized. What's more, the
outlook  for  growth in the major industrialized nations has been improving. The
domestic  economy continued to move ahead briskly, evidenced by a strong rebound
in  manufacturing  output,  which  shows  signs  of  gaining  momentum. Consumer
confidence  was  at  a  30-year  high.  Employment was strong, with hourly wages
rising.  Despite  concerns  that  overly  rapid  economic  growth  might lead to
destructive  inflationary  pressure,  the Fed held interest rates steady through
all  but the very end of the period. Because we managed the fund at a relatively
longer  average  maturity, investors have been able to benefit from stable rates
and the Fed's long-held accommodative stance during the reporting period.

What is the fund's current strategy?

Throughout  the  reporting period, as the economy showed robust growth, bond and
money  markets  anticipated  a  tightening  of  monetary policy by the Fed. Such
tightening  was  signaled by Chairman Alan Greenspan's mid-May announcement of a
shift  in  policy  towards  a bias to increase rates. The bias changed, in fact,
just    prior    to    the    end    of    the


<PAGE>


reporting  period,  when  the  Fed  raised  the  target  Federal  Funds rates by
one-quarter of a point to five percent, with an accompanying return to a neutral
stance  on  future Federal Funds rate movement. An initial relief rally has been
replaced by a cautious "wait and see" market view.

Over  the  reporting  period, the fund benefited from our commitment to a longer
maturity  structure.  When  rates  did  not rise as quickly as markets expected,
longer  maturities  enhanced  return. However, over the past few months, we have
taken a somewhat less aggressive stance, slowly reducing the average maturity of
our  investments.  In  an  uncertain  market  with  the  potential  for  further
tightening,  we have adopted this approach, while still seeking opportunities to
capture additional yield as such opportunities arise.

July 15, 1999

(1)  ANNUALIZED EFFECTIVE YIELD IS BASED UPON DIVIDENDS DECLARED DAILY AND
REINVESTED MONTHLY. PAST PERFORMANCE IS NO GUARANTEE OF FUTURE RESULTS. YIELDS
FLUCTUATE. AN INVESTMENT IN THE FUND IS NOT INSURED OR GUARANTEED BY THE FDIC OR
ANY OTHER GOVERNMENT AGENCY. ALTHOUGH THE FUND SEEKS TO PRESERVE THE VALUE OF
YOUR INVESTMENT AT $1.00 PER SHARE, IT IS POSSIBLE TO LOSE MONEY BY INVESTING IN
THE FUND.

(2)  TOTAL RETURN INCLUDES REINVESTMENT OF DIVIDENDS.

(3)  SOURCE: LIPPER ANALYTICAL SERVICES, INC.

                                                             The Fund

<PAGE>
<TABLE>
<CAPTION>
STATEMENT OF INVESTMENTS-- Money Market Series

June 30, 1999 (Unaudited)

                                                                                             Principal
NEGOTIABLE BANK CERTIFICATES OF DEPOSIT--8.1%                                                Amount ($)                Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------
<S>                                                                                          <C>                       <C>
Branch Bank & Trust Co.

   5.04%, 1/10/2000                                                                          18,000,000               17,997,257

First National Bank of Maryland

   5.14%, 2/23/2000                                                                          26,000,000               25,994,309

TOTAL NEGOTIABLE BANK CERTIFICATES OF DEPOSIT

   (cost $43,991,566)                                                                                                 43,991,566
- ------------------------------------------------------------------------------------------------------------------------------------

COMMERCIAL PAPER--42.9%

- ------------------------------------------------------------------------------------------------------------------------------------
Abbey National North America

   4.97%, 12/1/1999                                                                          15,670,000               15,348,334

BankAmerica Corp.

   4.96%, 10/7/1999                                                                          25,000,000               24,671,292

DaimlerChrysler NA Holdings

   4.92%, 7/9/1999                                                                           25,000,000               24,973,056

Donaldson, Lufkin & Jenrette Securities Corp.

   4.97%, 8/12/1999                                                                           5,000,000                4,971,358

Finova Capita1 Corp.

   4.96%-5.06%,  9/24/1999-10/22/1999                                                        26,800,000               26,413,570

General Electric Capital  Services Inc.

   4.97%, 10/20/1999                                                                         20,000,000               19,703,383

HSBC Americas Inc.

   4.99%, 11/24/1999                                                                         25,000,000               25,001,956

Hertz Corp.

   4.91%, 7/8/1999                                                                            5,000,000                4,995,285

Merita North America Inc.

   5.04%, 8/11/1999                                                                          25,000,000               24,857,639

Monsanto Co.

   4.94%, 10/19/1999                                                                          5,000,000                4,926,361

Paine Webber Group Inc.

   5.08%, 7/6/1999                                                                           17,000,000               16,988,313

Lehman Brothers Holdings Inc.

   5.40%, 8/5/1999                                                                           20,000,000               19,897,722

UBS Finance (DE) Inc.

   5.60%, 7/1/1999                                                                           21,000,000               21,000,000

TOTAL COMMERCIAL PAPER

   (cost $233,748,269)                                                                                               233,748,269


<PAGE>


                                                                                             Principal
CORPORATE NOTES--24.5%                                                                       Amount ($)                Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------

Bear Stearns Companies Inc.

   4.98%-5.09%, 9/15/1999-2/22/2000                                                          23,850,000  (a)          23,876,494

CIT Group Holdings Inc.

   4.87%, 9/21/1999                                                                          20,000,000  (a)          19,998,684

Ford Motor Credit Corp.

   4.95%, 5/5/2000                                                                           20,000,000  (a)          20,000,000

GTE Corp.

   5.20%, 6/12/2000                                                                          25,000,000  (a)          24,984,742

Heller Financial Inc.

   5.00%-5.01%, 9/8/1999-4/13/2000                                                           15,000,000  (a)          15,018,714

Paine Webber Group Inc.

   5.22%-5.32%, 10/4/1999-4/20/2000                                                           9,800,000  (a)           9,806,339

Salomon Smith Barney Holdings Inc.

   5.00%, 10/28/1999                                                                         20,000,000  (a)          20,000,000

TOTAL CORPORATE NOTES

   (cost $133,684,973)                                                                                               133,684,973
- ------------------------------------------------------------------------------------------------------------------------------------


SHORT-TERM BANK NOTES--20.1%
- ------------------------------------------------------------------------------------------------------------------------------------

First Tennessee Bank

   4.84%, 4/19/2000                                                                          20,000,000  (a)          19,993,722

First Union National Bank

   5.21%, 9/24/1999                                                                          25,000,000               25,000,000

Key Bank N.A.

   4.97%, 10/15/1999                                                                         25,000,000  (a)          25,002,877

Lasalle National Bank

   4.91%, 10/12/1999                                                                         20,000,000               20,000,000

Old Kent Bank & Trust Co.

   4.96%, 6/2/2000                                                                           20,000,000  (a)          19,989,343

TOTAL SHORT-TERM BANK NOTES

   (cost $109,985,942)                                                                                               109,985,942

                                                                                                     The Fund

<PAGE>


STATEMENT OF INVESTMENTS (Unaudited) (CONTINUED)

                                                                                             Principal
TIME DEPOSITS--3.6%                                                                          Amount ($)                Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------

Republic National Bank of New York (London)

  5.12%, 7/1/1999

   (cost $19,644,000)                                                                        19,644,000               19,644,000
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL INVESTMENTS

   (cost $541,054,750)                                                                            99.2%              541,054,750

CASH AND RECEIVEABLES (NET)                                                                         .8%                4,231,500

NET ASSETS                                                                                       100.0%              545,286,250

(A)  VARIABLE INTEREST RATE--SUBJECT TO PERIODIC CHANGE.

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>



<PAGE>
<TABLE>
<CAPTION>

STATEMENT OF INVESTMENTS--

Government Securities Series

June 30, 1999 (Unaudited)

                                                                          Annualized
                                                                           Yield on
                                                                            Date of              Principal

U.S.  TREASURY NOTES--55.3%                                              Purchase (%)            Amount ($)            Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------
<S>                                                                      <C>                     <C>                   <C>
6.375%, 7/15/1999                                                                5.35             5,000,000            5,001,536

5.875%, 7/31/1999                                                                4.52            19,000,000           19,014,394

5.875%, 8/31/1999                                                                4.71             5,000,000            5,008,246

5.875%, 2/15/2000                                                                4.78             8,000,000            8,051,223

5.5%, 3/31/2000                                                                  4.66             5,000,000            5,022,748

5.5%, 5/31/2000                                                                  4.88             5,000,000            5,017,954

5.875%, 6/30/2000                                                                5.23             3,000,000            3,014,959

TOTAL U.S. TREASURY NOTES

   (cost $50,131,060)                                                                                                 50,131,060
- ------------------------------------------------------------------------------------------------------------------------------------

REPURCHASE AGREEMENTS--43.4%
- ------------------------------------------------------------------------------------------------------------------------------------

Bear Stearns & Co.
   dated 6/30/1999, due 7/1/1999 in the amount of
   $8,001,666 (fully collateralized by
   $8,365,000 U.S. Treasury Strips,
   due 2/15/2000, value $8,055,495)                                              4.80             8,000,000            8,000,000

C.S. First Boston Corp.
   dated 6/30/1999, due 7/1/1999 in the amount of
   $5,000,666 (fully collateralized by
   $5,044,000 U.S. Treasury Notes, 5.625%,
   due 4/30/2000, value $5,098,911)                                              4.80             5,000,000            5,000,000

Donaldson Lufkin Jenrette Securities Corp. Inc.
   dated 6/30/1999, due 7/1/1999 in the amount of
   $8,001,066 (fully collateralized by
   $7,942,000 U.S. Treasury Notes, 7.875%
   due 11/15/1999, value $8,102,081)                                             4.80             8,000,000            8,000,000

Goldman Sachs & Co.
   dated 6/30/1999, due 7/1/1999 in the amount of
   $10,320,928 (fully collateralized by
   $3,950,000 U.S. Treasury Notes, 6.375%
   due 6/22/2000, value $10,590,330)                                             3.24            10,320,000           10,320,000

Morgan Stanley & Co.
   dated 6/30/1999, due 7/1/1999 in the amount of

   $8,001,048 (fully collateralized by
   $7,900,000 U.S. Treasury Notes, 6.875%,
   due 3/31/2000, value $8,121,445)                                              4.72             8,000,000            8,000,000

                                                                                                     The Fund

<PAGE>


STATEMENT OF INVESTMENTS (Unaudited) (CONTINUED)

                                                                       Annualized
                                                                         Yield on
                                                                          Date of

REPURCHASE AGREEMENTS (CONTINUED)                                        Purchase (%)                                  Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL REPURCHASE AGREEMENTS

   (cost $39,320,000)                                                                                                 39,320,000
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL INVESTMENTS (cost $89,451,060)                                            98.7%                                 89,451,060

CASH AND RECEIVABLES ( NET)                                                      1.3%                                  1,159,346

NET ASSETS                                                                     100.0%                                 90,610,406

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>

<PAGE>
<TABLE>
<CAPTION>

STATEMENT OF ASSETS AND LIABILITIES

June 30, 1999 (Unaudited)

                                                                                                      Money              Government
                                                                                                     Market              Securities
                                                                                                     Series                  Series
- ------------------------------------------------------------------------------------------------------------------------------------
<S>                                                                                                  <C>                  <C>
ASSETS ($):

Investments in securities, at value (including repurchase
   agreements of $39,320,000 for the Government
   Securities Series)--Note 2(b)                                                                 541,054,750             89,451,060

Cash                                                                                                 738,385                194,902

Interest receivable                                                                                3,795,429                984,904

Prepaid expenses                                                                                      19,272                 12,976

                                                                                                 545,607,836             90,643,842
- ------------------------------------------------------------------------------------------------------------------------------------

LIABILITIES ($):

Due to The Dreyfus Corporation and affiliates                                                        246,636                  7,780

Accrued expenses                                                                                      74,950                 25,656

                                                                                                     321,586                 33,436
- ------------------------------------------------------------------------------------------------------------------------------------

NET ASSETS ($)                                                                                   545,286,250             90,610,406
- ------------------------------------------------------------------------------------------------------------------------------------

COMPOSITION OF NET ASSETS ($):

Paid-in capital                                                                                  545,323,786             90,620,282

Accumulated net realized gain (loss) on investments                                                  (37,536)                (9,876)
- ------------------------------------------------------------------------------------------------------------------------------------

NET ASSETS ($)                                                                                   545,286,250             90,610,406
- ------------------------------------------------------------------------------------------------------------------------------------

SHARES OUTSTANDING

(unlimited number of $.001 par value shares of
   Beneficial Interest authorized)                                                               545,323,786             90,620,282

NET ASSET VALUE, OFFERING AND REDEMPTION PRICE PER SHARE ($)                                            1.00                   1.00

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>
                                                             The Fund

<PAGE>

<TABLE>
<CAPTION>
STATEMENT OF OPERATIONS

Six Months Ended June 30, 1999 (Unaudited)

                                                            Money   Government
                                                           Market   Securities
                                                           Series       Series
- -----------------------------------------------------------------------------------
<S>                                                        <C>      <C>
INVESTMENT INCOME ($):

INTEREST INCOME                                        14,272,181    1,905,932

EXPENSES--NOTE 2(C):

Management fee--Note 3(a)                               1,410,213      200,719

Shareholder servicing costs--Note 3(b)                     72,863        1,883

Custodian fees                                             34,744       14,256

Trustees' fees and expenses--Note 3(c)                     31,083        5,924

Registration fees                                          17,728        9,849

Professional fees                                          16,682       20,574

Prospectus and shareholders' reports                          219           --

Miscellaneous                                              19,732          857

TOTAL EXPENSES                                          1,603,264      254,062

INVESTMENT INCOME--NET                                 12,668,917    1,651,870
- -----------------------------------------------------------------------------------

NET REALIZED GAIN (LOSS) ON INVESTMENTS--NOTE 2(B)($):     (3,181)      (9,876)

NET INCREASE IN NET ASSETS RESULTING FROM OPERATIONS   12,665,736    1,641,994

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>

<PAGE>
<TABLE>
<CAPTION>

STATEMENT OF CHANGES IN NET ASSETS

                                              Money Market Series                          Government Securities Series
                                   -------------------------------------------   ----------------------------------------------

                                   Six Months Ended                               Six Months Ended
                                      June 30, 1999              Year Ended          June 30, 1999                Year Ended
                                        (Unaudited)       December 31, 1998             (Unaudited)        December 31, 1998
- ------------------------------------------------------------------------------------------------------------------------------------
<S>                                 <C>                   <C>                      <C>                      <C>
OPERATIONS:

Investment income--
   net                                   12,668,917              27,102,277              1,651,870                  3,992,120

Net realized gain (loss)
   on investments                            (3,181)                (27,774)                (9,876)                    10,367

NET INCREASE (DECREASE)
   IN NET ASSETS
   RESULTING FROM
   OPERATIONS                            12,665,736              27,074,503              1,641,994                  4,002,487

DIVIDENDS TO
   SHAREHOLDERS
   FROM ($):

Investment income--
   net                                 (12,668,917)             (27,102,277)            (1,651,870)                (3,992,120)

Net realized gain on
   investments                                 --                        --                    --                     (10,396)

TOTAL DIVIDENDS                        (12,668,917)             (27,102,277)            (1,651,870)                (4,002,516)

BENEFICIAL INTEREST
   TRANSACTIONS
   ($1.00 per share) ($):

Net proceeds from
   shares sold                        1,762,978,510           3,374,356,270            395,819,514                579,548,799

Dividends reinvested                      1,400,367               2,871,717                680,882                  1,501,989

Cost of shares
   redeemed                          (1,759,371,651)         (3,368,354,264)           (374,495,030)             (620,130,251)

INCREASE (DECREASE)
   IN NET ASSETS FROM
   BENEFICIAL INTEREST
   TRANSACTIONS                           5,007,226               8,873,723              22,005,366               (39,079,463)

TOTAL INCREASE
   (DECREASE) IN
   NET ASSETS                             5,004,045               8,845,949              21,995,490               (39,079,492)

NET ASSETS

Beginning of Period                     540,282,205             531,436,256              68,614,916               107,694,408

END OF PERIOD                           545,286,250             540,282,205              90,610,406                68,614,916

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>
                                                             The Fund

<PAGE>
<TABLE>
<CAPTION>
FINANCIAL HIGHLIGHTS--Money Market Series

The  following tables describe the performance for the fiscal periods indicated.
Total  return  shows  how  much your investment in the fund would have increased
(or decreased) during each period, assuming you had reinvested all dividends and
distributions.  These  figures  have  been  derived  from  the  fund's financial
statements.

                                          Six Months Ended
                                             June 30, 1999                               Year Ended December 31,
                                                                    ----------------------------------------------------------------
                                                (Unaudited)         1998         1997          1996           1995          1994
- ------------------------------------------------------------------------------------------------------------------------------------
<S>                                         <C>                    <C>           <C>           <C>            <C>           <C>
PER SHARE DATA ($):

Net asset value,

   beginning of period                                1.00          1.00          1.00           1.00          1.00          1.00

Investment Operations:

Investment income--net                                .022           .050          .051           .049          .054          .036

Distributions:

Dividends from investment

   income--net                                       (.022)         (.050)        (.051)         (.049)        (.054)        (.036)

Net asset value, end of period                       1.00           1.00          1.00           1.00          1.00          1.00
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL RETURN (%)                                     4.54*          5.14          5.17           5.03          5.57          3.65

- ------------------------------------------------------------------------------------------------------------------------------------

RATIOS/SUPPLEMENTAL DATA (%):

Ratio of expenses to
   average net assets                                 .57*           .57           .57            .58           .62          .63

Ratio of net investment income

   to average net assets                             4.49*          5.02          5.06           4.91          5.43         3.59
- ------------------------------------------------------------------------------------------------------------------------------------

Net Assets, end of period
   ($ x 1,000)                                     545,286       540,282       531,436        483,156       401,032      362,825

*  ANNUALIZED.

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>

<PAGE>
<TABLE>
<CAPTION>
FINANCIAL HIGHLIGHTS--Government Series

                                          Six Months Ended
                                             June 30, 1999                               Year Ended December 31,
                                                                    ----------------------------------------------------------------

                                                (Unaudited)         1998         1997          1996           1995          1994
- ------------------------------------------------------------------------------------------------------------------------------------
<S>                                         <C>                    <C>           <C>           <C>            <C>           <C>
PER SHARE DATA ($):

Net asset value,

   beginning of period                                1.00          1.00          1.00           1.00          1.00          1.00

Investment Operations:

Investment income--net                                 .020          .048          .048           .047          .052          .034

Distributions:

Dividends from investment

   income--net                                        (.020)        (.048)        (.048)         (.047)        (.052)        (.034)

Net asset value, end of period                        1.00          1.00          1.00           1.00          1.00          1.00
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL RETURN (%)                                     4.15*          4.87          4.95           4.84          5.36          3.49
- ------------------------------------------------------------------------------------------------------------------------------------

RATIOS/SUPPLEMENTAL DATA (%):

Ratio of expenses to
   average net assets                                 .63*           .68           .61            .63           .65           .69

Ratio of net investment income

   to average net assets                             4.11*          4.76          4.84           4.74          5.23          3.40
- ------------------------------------------------------------------------------------------------------------------------------------

Net Assets, end of period
   ($ x 1,000)                                      90,610        68,615       107,694        102,726        123,171      120,281

*  ANNUALIZED.

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>
                                                             The Fund

<PAGE>


NOTES TO FINANCIAL STATEMENTS (Unaudited)

NOTE 1--General:

Dreyfus  Institutional  Money  Market  Fund (the "fund") is registered under the
Investment  Company  Act  of  1940,  as  amended  (the  "Act"), as a diversified
open-end  management investment company and operates as a series company issuing
two  classes  of Beneficial Interest: the Money Market Series and the Government
Securities  Series. The fund accounts separately for the assets, liabilities and
operations  of  each  series.  The  fund' s  investment  objective is to provide
investors  with  as  high  a  level  of current income as is consistent with the
preservation   of   capital  and  the  maintenance  of  liquidity.  The  Dreyfus
Corporation (the "Manager") serves as the fund's investment adviser. The Manager
is  a  direct subsidiary of Mellon Bank, N.A. Premier Mutual Fund Services, Inc.
is  the distributor of the fund's shares, which are sold to the public without a
sales charge.

It  is  the  fund's policy to maintain a continuous net asset value per share of
$1.00  for  each  series;  the  fund  has  adopted certain investment, portfolio
valuation and dividend and distribution policies to enable it to do so. There is
no assurance, however, that the fund will be able to maintain a stable net asset
value    per    share    of    $1.00    for    each    series.

The  funds'  financial  statements  are  prepared  in  accordance with generally
accepted accounting principles which may require the use of management estimates
and assumptions. Actual results could differ from those estimates.

NOTE 2--Significant Accounting Policies:

(a) Portfolio valuation: Investments in securities are valued at amortized cost,
which has been determined by the fund's Board of  Trustees to represent the fair
value of the fund's investments.

(b)  Securities  transactions and investment income: Securities transactions are
recorded  on  a  trade  date  basis.  Realized  gain  and  loss  from securities
transactions  are  recorded  on  the  identified  cost basis. Interest income is
recognized  on the accrual basis. Cost of investments represents amortized cost.
Under    the    terms    of    the    custody    agreement,

<PAGE>


the  Money  Market  Series  receives net earning credits based on available cash
balances  left  on  deposit.  Under  the  terms  of  the  custody agreement, the
Government  Securities Series received net earnings credits of $1,112 during the
period  ended  June 30, 1999 based on available cash balances on deposit. Income
earned    under    this    arrangement    is    included    in    interest.

The  fund  may  enter  into  repurchase  agreements with financial institutions,
deemed  to  be  creditworthy  by  the  fund' s  Manager, subject to the seller's
agreement  to repurchase and the fund's agreement to resell such securities at a
mutually   agreed   upon  price.  Securities  purchased  subject  to  repurchase
agreements are deposited with the fund's custodian and, pursuant to the terms of
the  repurchase  agreement,  must have an aggregate market value greater than or
equal  to  the repurchase price plus accrued interest at all times. If the value
of  the underlying securities falls below the value of the repurchase price plus
accrued  interest,  the  fund  will  require  the  seller  to deposit additional
collateral by the next business day. If the request for additional collateral is
not met, or the seller defaults on its repurchase obligation, the fund maintains
the  right  to  sell the underlying securities at market value and may claim any
resulting loss against the seller.

(c)  Expenses: Expenses directly attributable to each series are charged to that
series'  operations;  expenses which are applicable to both series are allocated
among them on a pro rata basis.

(d)  Dividends  to  shareholders:  It is the policy of the fund, with respect to
both  series,  to  declare dividends from investment income-net on each business
day;  such dividends are paid monthly. Dividends from net realized capital gain,
with  respect  to both series, are normally declared and paid annually, but each
series  may  make  distributions  on  a  more  frequent basis to comply with the
distribution  requirements of the Internal Revenue Code of 1986, as amended (the
" Code" ). However,  to  the  extent  that a net realized capital gain of either
series can be reduced by a capital loss carryover of that series, such gain will
not be distributed.

                                                             The Fund

<PAGE>


NOTES TO FINANCIAL STATEMENTS (Unaudited) (CONTINUED)

(e) Federal income taxes: It is the policy of each series to continue to qualify
as  a  regulated  investment  company,  if  such  qualification  is  in the best
interests  of  its  shareholders, by complying with the applicable provisions of
the  Code,  and to make distributions of taxable income sufficient to relieve it
from substantially all Federal income and excise taxes.

The  Money  Market  Series has an unused capital loss carryover of approximately
$34,500  available  for Federal income tax purposes to be applied against future
net securities profits, if any, realized subsequent to December 31, 1998. If not
applied,  $6,500  of the carryover expires in fiscal 2002 and $28,000 expires in
2006.

At  June 30, 1999, the cost of investments of each series for Federal income tax
purposes was substantially the same as the cost for financial reporting purposes
(see the Statements of Investments).

NOTE 3--Management Fee and Other Transactions With Affiliates:

(a)  Pursuant to a management agreement with the Manager, the management fee for
each  series  is  computed  at  the annual rate of .50 of 1% of the value of the
average daily net assets of each series and is payable monthly.

(b)  Under the Shareholder Services Plan, each series reimburses Dreyfus Service
Corporation,  a  wholly-owned subsidiary of the Manager, an amount not to exceed
an  annual  rate  of  .25  of  1% of the value of each series' average daily net
assets  for  certain  allocated  expenses  of providing personal services and/or
maintaining  shareholder  accounts.  The  services provided may include personal
services  relating  to  shareholder  accounts,  such  as  answering  shareholder
inquiries  regarding  the  fund and providing reports and other information, and
services  related  to the maintenance of shareholder accounts. During the period
ended  June  30,  1999,  the  Money  Market Series and the Government Securities
Series   were  charged  $29,818  and  $20,246,  respectively,  pursuant  to  the
Shareholder Services Plan.


<PAGE>


The  fund  compensates  Dreyfus Transfer, Inc., a wholly-owned subsidiary of the
Manager,   under  a  transfer  agency  agreement  for  providing  personnel  and
facilities  to  perform transfer agency services for the fund. During the period
ended  June  30,  1999,  the  Money  Market Series and the Government Securities
Series  were  charged $47,456 and $1,950, respectively, pursuant to the transfer
agency agreement.

(c)  Each  trustee  who  is  not  an  "affiliated  person" as defined in the Act
receives from the fund an annual fee of $4,500 and an attendance fee of $500 per
meeting.  The  Chairman  of  the  Board  receives  an  additional  25%  of  such
compensation.

                                                             The Fund

<PAGE>


NOTES

<PAGE>


                                                           For More Information

                        Dreyfus

                        Institutional

                        Money Market Fund

                        200 Park Avenue

                        New York, NY 10166

                        Manager

                        The Dreyfus Corporation

                        200 Park Avenue

                        New York, NY 10166

Custodian

The Bank of New York

90 Washington Street

New York, NY 10286

Transfer Agent &

Dividend Disbursing Agent

Dreyfus Transfer, Inc.

P.O. Box 9671

Providence, RI 02940

Distributor

Premier Mutual Fund Services, Inc.

60 State Street

Boston, MA 02109

To obtain information:

BY TELEPHONE Call 1-800-645-6561

BY MAIL  Write to:  The Dreyfus Family of Funds 144 Glenn Curtiss Boulevard
Uniondale, NY 11556-0144

BY E-MAIL  Send your request to [email protected]

ON THE INTERNET  information can be viewed online or downloaded from:

http://www.dreyfus.com

(c) 1999 Dreyfus Service Corporation                              179/195SA996



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